Passing function as argument in F# - f#

Question

In the following example, I'm receiving the error "No overloads match for method 'op_Subtraction'."
open System
type EmployeeStatus =
| Active
| NotActive
type EnrollmentPeriod =
| JanuaryFirst
| JulyFirst
| PeriodNotApplicable
let DetermineTargettedEnrollmentDate targettedEnrollmentDate (relativeDate : DateTime) =
match targettedEnrollmentDate with
| EnrollmentPeriod.JanuaryFirst -> new DateTime(relativeDate.Year + 1,1,1)
| EnrollmentPeriod.JulyFirst -> new DateTime(relativeDate.Year,7,1)
| EnrollmentPeriod.PeriodNotApplicable -> relativeDate
let ProjectDaysWorkedSinceJan1 employeeStatus targettedEnrollmentPeriod (relativeDate : DateTime) =
let januaryFirst = DateTime(relativeDate.Year,1,1)
let targettedEnrollmentDate = DetermineTargettedEnrollmentDate targettedEnrollmentPeriod
match employeeStatus with
| Active -> int (januaryFirst - targettedEnrollmentDate).TotalDays
| NotActive -> 0
It doesn't appear to like that targettedEnrollmentDate is being determined by the DetermineTargettedEnrollmentDate function.
Where am I going wrong here?

Answer 1

You only supply DetermineTargettedEnrollmentDate with the first argument:
let targettedEnrollmentDate = DetermineTargettedEnrollmentDate targettedEnrollmentPeriod
The result of that is a function waiting for the second argument, i.e. targettedEnrollmentDate is a function that takes relativeDate as an argument.
So, what you try to do, is subtract a function from a date and that's what the compiler complains about.

Answer 2

I don't really understand your question. The code clearly doesn't compile.
DetermineTargettedEnrollmentDate is defined as
val DetermineTargettedEnrollmentDate :
targettedEnrollmentDate:EnrollmentPeriod ->
relativeDate:DateTime -> DateTime
Which means, it takes two parameters: an EnrollmentPeriod and a DateTime and returns another DateTime. Later in ProjectDaysWorkedSinceJan1 you use it with just one argument:
let targettedEnrollmentDate = DetermineTargettedEnrollmentDate targettedEnrollmentPeriod
Because of partial application, this makes targettedEnrollmentDate a function, which takes DateTime and return another DateTime. But later you try to use it as if it was a value, when you try to substract another DateTime from it.
| Active -> int (januaryFirst - targettedEnrollmentDate).TotalDays
----------------------------------^
(...)
Possible overload: 'DateTime.op_Subtraction(d1: DateTime, d2: DateTime) : TimeSpan'. Type constraint mismatch. The type
DateTime -> DateTime
is not compatible with type
DateTime
What you probably want is to call DetermineTargettedEnrollmentDate with relativeDate instead:
let ProjectDaysWorkedSinceJan1 employeeStatus targettedEnrollmentPeriod (relativeDate : DateTime) =
let januaryFirst = DateTime(relativeDate.Year,1,1)
let targettedEnrollmentDate = DetermineTargettedEnrollmentDate targettedEnrollmentPeriod relativeDate
match employeeStatus with
| Active -> int (januaryFirst - targettedEnrollmentDate).TotalDays
| NotActive -> 0
Which makes it compile with following result:
val ProjectDaysWorkedSinceJan1 :
employeeStatus:EmployeeStatus ->
targettedEnrollmentPeriod:EnrollmentPeriod -> relativeDate:DateTime -> int